Continuous Leaching Approach for Gold Recovery from Printed Circuit Boards using Hexaamminecobalt(III)–Thiosulfate Generated from Spent Lithium Cobalt Oxide

A significant number of end-of-life computer central processing units are annually discarded, with many printed circuit boards (PCBs) containing valuable metals such as Au and Ni. This study focuses on a “continuous leaching” system and introduces an innovative method. It utilizes ammonia leaching and acid leaching sequentially to process spent lithium cobalt oxide (LCO) powder (referred to as LCO powder). The resulting leachate is then utilized as the cobalt source for a subsequent continuous leaching process to directly synthesize the [Co(NH3)6]3+ complex. Finally, the [Co(NH3)6]3+-thiosulfate system is used for gold leaching from waste PCBs. Experimental results indicate that optimal conditions for ammonia leaching–2.5 mol/L (NH4)2SO3, 0.5 mol/L NH3·H2O, 0.5 mol/L (NH4)2CO3, 80°C temperature, 100 : 1 mL/g liquid-to-solid ratio, and 10-h leaching time–yield leaching efficiencies of 84.14 and 87.69% for Co and Li, respectively. For acid leaching, optimal conditions include 0.5 mol/L H2SO4, 2% H2O2, 80°C temperature, 100 : 3 mL/g liquid-to-solid ratio, and 2-h leaching time, resulting in leaching efficiencies of 95.29 and 70.69% for Co and Li, respectively. The leachate from acid leaching is then used to synthesize the [Co(NH3)6]3+ complex, followed by employing the [Co(NH3)6]3+-thiosulfate system for gold leaching, achieving a gold leaching efficiency of 95.53% under optimal conditions. This study utilizes spent LCO powder to produce the [Co(NH3)6]3+ complex instead of directly using [Co(NH3)6]3+ salts (e.g., [Co(NH3)6]Cl3), resulting in a significant cost reduction, decreased reagent consumption, and showcasing two key innovations: “synergistic extraction” and “continuous leaching”.